The present invention relates to methods and compositions for treating subterranean formations. More particularly, the present invention relates to polymersomes, viscosifying agents that comprise polymersomes, and associated methods of use.
Viscosified treatment fluids may be used in a variety of operations and treatments performed in oil and gas wells. Such operations and treatments include, but are not limited to, drilling operations, production stimulation operations, and completion operations.
Drilling processes typically require the use of a drilling fluid. During drilling operations, the drilling fluid passes down through the inside of the drill string, exits through the drill bit, and returns to the drilling rig through the annulus between the drill string and well bore. The circulating drilling fluid, inter alia, lubricates the drill bit, carries drill cuttings to the surface, and balances the formation pressure exerted on the well bore. A drill-in fluid is a specially designed drilling fluid that is used for drilling into the production zone of a subterranean formation. Drilling fluids typically require sufficient viscosity to suspend drill cuttings.
Well stimulation treatments, such as fracturing treatments, commonly employ viscosified treatment fluids. Fracturing operations generally involve pumping a viscous fracturing fluid into a subterranean formation at a sufficient hydraulic pressure to create or enhance one or more cracks or “fractures” in the subterranean formation. The fracturing fluid generally has a viscosity sufficient to carry particulates (e.g., proppant) to at least one fracture, inter alia, to aid in maintaining the integrity of those fractures once the hydraulic pressure is released. Once at least one fracture is created and the proppant is substantially in place, the viscosity of the fracturing fluid usually is reduced, and the fluid is recovered from the formation.
Similarly, completion operations (e.g., gravel packing) also employ viscosified treatment fluids. Gravel packing operations typically use viscosified gravel pack fluids to suspend gravel particulates for delivery to a desired area in a well bore, e.g., near unconsolidated or weakly consolidated formation zones. One common type of gravel packing operation involves placing a gravel pack screen in the well bore and packing the annulus between the screen and the well bore with particulates (e.g., gravel) of a specific size designed to prevent the passage of formation sand. When installing the gravel pack, oftentimes the particulates are carried to the annulus behind the screen in the form of a slurry by mixing the gravel particulates with a viscosified treatment fluid. The particulates act, inter alia, to prevent the formation particulates from occluding the screen or migrating with the produced fluids, and the screen acts, inter alia, to prevent the particulates from entering the production tubing. Once the gravel pack is substantially in place, the viscosity of the gravel pack fluid often is reduced to allow it to be recovered.
Viscosified treatment fluids are used, inter alia, in drilling operations, fracturing operations, and gravel pack operations. To accomplish tasks such as those described above, having a treatment fluid with the appropriate rheological characteristics (e.g., viscosity, yield point, etc.) may be critical. In drilling operations, yield point of the drilling fluid may be an important characteristic. Yield point is used, inter alia, to determine the ability of a drilling fluid to carry drill cuttings out of the well bore, generally speaking, the higher a yield point the better the suspension capabilities of the drilling fluid. To obtain the desired yield points (as well as other flow properties), drilling fluids typically comprise complicated emulsions that comprise clays and other solids. But these emulsions often result in increased expense and complexity in the drilling operation.
Another important characteristic in the viscosified treatment fluids is viscosity. Among other reasons, the viscosified treatment fluids should have a viscosity sufficient to transport solids, such as particulates or drill cuttings. Furthermore, maintaining sufficient viscosity, also is important, in fracturing operations, to create or enhance fracture width. To modify the treatment fluids to obtain optimum properties, viscosifying agents commonly are added to treatment fluids, e.g., to provide fluid loss control and suspend particulates, such as proppant or gravel. Examples of commonly used viscosifying agents include, but are not limited to, guar gum, guar derivatives, hydroxyethyl cellulose, xanthan, starches, polyacrylamide, and other conventional viscosifying agents. The use of conventional viscosifying agents, however, may be problematic. For instance, these viscosifying agents may leave an undesirable chemical residue in the subterranean formation after use. As a result, potentially costly remedial operations may be required to clean up the well bore. Further, to minimize residual damage, foamed treatment fluids and emulsion-based treatment fluids have been employed, but increased expense and complexity often result.